This invention relates to a stud welding apparatus for applying and welding studs to a workpiece such as, for example, an automotive body part. More specifically, the invention relates to an improved mechanism for automatically adjusting the stroke of the lifting mechanism to compensate for studs of differing length while maintaining a desired lift height from the workpiece.
In the process of attaching studs to workpieces by means of an electric arc, solenoid and spring systems are used to lift the stud away from contact with the workpiece, thus producing an arc, and to plunge the stud into a pool of molten metal produced after an interval by the arc. One of the critical parameters in this process is the lift height which is the maximum distance between the tip of the stud and the surface of the workpiece. If a fixed connection is provided between the stud carrier and the solenoid-spring driving mechanism, difficulties can arise in establishing the accurate lift height when studs of differing length are to be positioned in the stud carrier. In order to accommodate this situation, it is known to provide a clutch mechanism between an actuating pin which is directly connected to the solenoid and a dragging sleeve which is connected to the stud carrier. As the welding apparatus is lowered toward the workpiece to establish initial contact between the stud and the workpiece, the clutch enables the dragging sleeve to be disconnected from the actuating pin as the stud contacts the workpiece, even though the actuating pin may continue to travel a short distance beyond the point of stud contact. When the solenoid is initiated to lift the stud away from the workpiece, the clutch produces engagement between the actuating pin and dragging sleeve at an appropriate point in the stroke so that the dragging sleeve, stud carrier and stud are lifted to precisely the desired lift height away from the workpiece, thus enabling the desired weld to be achieved.
In known apparatus of this type, the stroke compensating arrangement has a clamping member which cooperates with the dragging sleeve and a conically shaped region of the actuating pin capable of reciprocating in its longitudinal direction. With known stroke compensating arrangements, the clamping member is spherical in design. The spherical clamping member is arranged within a duct in a holding member. The spherical clamping member rests on a conically shaped region of an actuating pin. The actuating pin is capable of reciprocating in its longitudinal direction. During the movement of the actuating pin, the spherical clamping member slides on an external surface of a conically shaped region of the actuating pin so the spherical clamping member performs a movement extending substantially perpendicularly to the movement of the actuating pin. The dragging sleeve surrounding the clamping member is arranged at such a distance therefrom that the spherical clamping member is clamped between the conical region and the dragging sleeve by a movement of the actuating pin in one direction creating a rigid connection between the dragging sleeve, the clamping member and the actuating pin. If the actuating pin is moved in an opposing direction, the clamping member is released by the conically shaped region. The rigid connection between the dragging sleeve, the clamping member and the actuating pin is therefore eliminated.
With such a design of a stroke compensating arrangement, a substantially spot-shaped connection is created between the clamping member and the dragging sleeve or the conically shaped region of the movable actuating pin. The forces which can therefore be transmitted by such a system of a stroke compensating arrangement are relatively low. There is also the risk that the dragging sleeve can be slipped through owing to the relatively small area of contact between the clamping member and the conically shaped region of the actuating pin or of the dragging sleeve. The risk of slipping through is increased by wear of the components, so exact stroke compensation cannot be ensured.